The use of refined indigo naturalis (indigo naturalis extract in oil, INEO)ointment to treat psoriasis has been proven effective in our previous study. This study aims to evaluate the efficacy and safety of INEO ointment, and further determine the optimal concentration of INEO ointment (per gram of ointment containing either 200 μg, 100 μg, 50 μg or 10 μg of indirubin) in treatment of various local skin signs and thickness of psoriasis plaque.